1. Field of the Invention
This invention relates to gas turbine engine fan sections, and particularly to fan cases and assemblies.
2. Description of Related Art
Gas turbine engines having multiple fan stage rotors are conventionally circumscribed by metal fan cases supporting at least one array of fan stators between the fan rotor stages. Metal fan cases are often made of titanium or steel alloys and provide fan blade containment in case a fan blade, or a part of a blade, breaks and is thrown radially outward into the case. Typically, multistage fan engines employ horizontally split fan cases around the rotor for ease of assembly, disassembly, reassembly, overhaul, repair, access to the rotor and balancing of the rotor, just to name a few advantages.
Horizontally split cases require yet thicker metal casings than are otherwise required in order to provide blade containment capabilities. This further adds to the weight of the casing which is very undesirable, particularly for high performance military type turbofan engines. Single fan stage engines of the type used primarily for commercial applications conventionally employ Kevlar-wrapped non-split or continuous ring cases. Continuous Kevlar wraps around fan cases is a well-known means of containing blade and blade fragments and preventing further engine or aircraft damage.
One can readily see the difficulty of trying to employ continuous rings to surround a multistage fan rotor. Conventionally, such assemblies employ circumferential flanges at the beginning and end of each ring case which are then bolted together, instead of employing a single ring case. Conventional multistage fan cases do not use Kevlar wraps, are made of metal, and have a heavy continuous ring and/or horizontally split cases. Kevlar is not used for split cases because of the discontinuity at the split line and its corresponding horizontal flange.
Another problem posed by composite material advances is how to form and mount lightweight composite vanes in a manner that will efficiently redistribute the loads from the vanes into the cases, while preventing delamination or other damage of composite vanes and cases. This problem is compounded by the complexity of high performance vane shapes, and in particular, the great degree of twist and curvature of the vanes.